Document pattern for "backing resources"

[ktheory]

Add examples for how to use cfn-flow with backing resources.

Backing resources are re-used across deploys, and may include:

• security groups
• IAM roles & instance profiles
• RDS instances

[snakescott]

I ran across the cfn-flow blog post and this was one of my first questions. Specifically, I would be interested in what best practices cfn-flow has for handling changes to stateful backing resources -- for example, change the instance type of an RDS instance.